A known printing apparatus for printing roll-to-roll comprises an input station on a front side of the printing apparatus, an output station on a back side of the printing apparatus and a printing station. The output station comprises one roll-up unit for receiving a print substrate from the printing station. In a first step of a known duplex printing process the print substrate is fed from a roll in the input station to a printing station for printing a first image on a first side of the print substrate. The print substrate, bearing the first image, is received in the output station. The print substrate is manually attached to the roll-up unit by the operator and is thereafter wind in the roll-up unit in the output station thereby forming a roll of the print substrate. In case of a need for printing duplex on the print substrate, the roll, bearing the first image, is unloaded from the roll-up unit in the output station by an operator and reloaded into the input station. In a second step the print substrate, bearing the first image, is fed from the input station to the printing station for printing a second image on a second side of the print substrate. The print substrate, bearing the first image and the second image, is received on the roll-up unit in the output station thereby forming a roll of the print substrate. The unloading and loading of the roll of the print substrate in the known printing apparatus interrupts the printing operation of the printing apparatus and reduces the productivity of the duplex output. For more productive roll-to-roll printing a need exists for automatic duplex printing without interruption of the duplex printing operation and a possibility of unloading a roll from the output station independent of the printing operation of the printing apparatus.